1. Field of the Invention
The present invention relates to a sheet supplying apparatus for a copying machine, a printer and the like, and more particularly, it relates to a sheet supplying apparatus of retard roller type which is cheap and simple and which has good sheet supplying and separating ability.
2. Related Background Art
In conventional sheet supplying apparatus of a so-called retard roller type wherein sheets stacked in a sheet supply cassette are separated and supplied one by one by three kinds of rollers comprising a sheet supply roller, a normal rotation roller and a reversible retard roller to feed the separated sheet to an image forming station, it is necessary to provide a lifter mechanism for maintaining a height of a top surface of the sheet stack substantially constant. The lifter mechanism is an effective means not only in the sheet supplying apparatus of retard roller type but also in sheet supplying apparatuses of other types, since a contact pressure between the sheet supply roller and the-sheet stack can be easily kept constant regardless of a thickness of the sheet stack by utilizing a weight of the roller itself and the like.
Now, the conventional sheet supplying apparatus will be explained with reference to FIG. 18.
In FIG. 18, the reference numeral 1 denotes an image forming apparatus such as a copying machine, a printer and the like; 2 denotes a sheet supply cassette; 3 denotes an intermediate plate pivotable around shafts 2a fixed to the sheet supply cassette 2; and S denotes a stack of transfer sheets. The reference numeral 4 denotes a height detection sensor for measuring a height of a top surface of the sheet stack, wherein movement of a lever made of resin such as polycarbonate is detected by a photo-interrupter, for example.
A lifter plate 5 serves to lift the intermediate plate 3 and is fixed to a lifter shaft 6 rotated by a lifter drive motor 6a. A sheet supply roller 7 can be rotated in a direction shown by the arrow and can be shifted in an up-and-down direction. The reference numeral 8 denotes a normal rotation drive roller; 9 denotes a retard roller which includes a torque limiter in its driving system; and 10 and 11 denote a pair of feed rollers.
Next, an operation of the sheet supplying apparatus will be briefly explained.
When a sheet supply signal is emitted, the sheet supply roller 7 is lowered to abut against the sheet stack and is rotated to supply an uppermost sheet of the sheet stack. Then, the supplied sheet enters between the normal rotation drive roller 8 and the retard roller 9. In this case, if a single sheet enters between the normal rotation drive roller and the retard roller, the retard roller 9 is rotated in a sheet feeding indirection; whereas, if two or more sheets enter between the normal rotation drive roller and the retard roller, the retard roller 9 is rotated reversely by the action of the torque limiter to successively return the sheet from the lowermost one toward the sheet supply cassette, thereby preventing the double-feed of the sheets. Further, the sheet supply roller 7 is shifted upwardly when a predetermined time is elapsed after the leading ends of the sheets have entered between the normal rotation drive roller 8 and the retard roller 9, so that the returned sheets can be smoothly returned back toward the sheet supply cassette.
In this way, by supplying the sheets successively from the uppermost one, a height of the top surface of the sheet stack is gradually decreased. When the fact that the height of the top surface of the sheet stack is decreased below a predetermined height is detected by the height detection sensor 4, the lifter drive motor 6a is rotated by a certain short time to lift the intermediate plate 3, thereby lifting the top surface of the sheet stack up to the predetermined level.
By repeating the above operations, the sheets can be successively supplied one by one.
However, in the above-mentioned conventional sheet supplying apparatus of retard roller type, as mentioned above, since the lifter mechanism 5, 6, 6a for the intermediate plate 3, the height detection sensor 4 and the mechanism for lowering and lifting the sheet supply roller 7 are required, the apparatus becomes considerably expensive and complicated in comparison with sheet supplying apparatuses of separation claw type and of dupro type, and has an disadvantage in a spatial problem, whereby it could not be adopted to a compact copying machine, a compact printer and the like.